Spread spectrum techniques have proven useful in a variety of communications applications, including cellular telephones, wireless local area networks, and military communications. Spread spectrum techniques provide a number of advantages.
Wireless spread spectrum systems operate by using a relatively large amount of spectrum bandwidth to communicate their signals. The large bandwidth is consumed by spread spectrum encoding the message data using a pseudonoise code. The two most common types of spread spectrum transmission are frequency hopping, where the pseudonoise code is used to pseudo randomly change the transmission frequency on a periodic basis, and direct sequence, where the pseudonoise code is used to modulate the transmit signal at a high rate relative to the underlying message data.
In order to detect a spread spectrum transmission, it is generally necessary to know the pseudonoise code beforehand. Furthermore, to extract the message data, it is generally necessary to know the timing of the pseudonoise code. For example, in a direct sequence system, this can be accomplished by knowing the code frequency (rate at which the pseudonoise code advances through its code) and the starting time of the pseudonoise code (sometimes referred to as the phase of the code). A signal for which the spread spectrum receiver knows the pseudonoise code, pseudonoise code phase, and pseudonoise code frequency can be referred to as a synchronized signal.
Achieving synchronization with a spread spectrum signal can be difficult, in part due to high pseudonoise code rate (frequency). To accurately demodulate the spread spectrum signal, it is generally necessary to know the pseudonoise code phase within a fraction of a single chip of the pseudonoise code. Typically, performing initial acquisition of a spread spectrum signal is a time consuming process.
In some situations, it is desirable for a system to transition between operating in a spread spectrum mode and an unspread (clear) mode. Unfortunately, when transitioning from unspread mode back into spread mode, a re-acquisition is often necessary to reestablish synchronization of the pseudonoise code. Lengthy re-acquisition time may be undesirable.